To avoid building an SBR it must have a bbl length of at least 16" and an overall length over 26" with the buttstock extended (if folding). Some states, notably Michigan, have their own regs regarding folding stocks and such.

Heavy enough bolt and recoil spring will make .45 work fine in blowback. All the subguns chambered in .45 have been blowback with the notable exception of the first Tommy guns. Even those were eventually converted to blowback operation with the M1 model.

It just has to have a barrel over 16 inches measured by putting a wood dowel down the bore with the bolt closed. OAL is 26". As far as I know the .45 doesnt have enough umph to operate with a gas system or at least very few examples exist because of practicality. Keep us posted on how its going.

Would the bolt need to be heavier for a long barrel like that? You want the bullet to get out the barrel before the bolt chambers another round right? This is my biggest worry. Not having the bolt and barrel length knowledge.

I will of course do some more research befroe I get that far into design. I only have some basic design thoguhts right now.

As long as it wasn't a squib load, the bullet should be out of the barrel before the bolt starts moving backwards.

For some numbers, a Thompson bolt weighs just over a pound and a M3 Grease Gun bolt weighs 2 pounds. Both use fairly light recoil springs. I have a Mech-Tech CCU with a 1911 lower that also uses a 2 pound bolt and a 16" barrel. You can play with spring rates and buffers to come up with a system that doesn't batter itself to pieces.

p.s. If you're going with a .45 I'd suggest picking a higher capacity mag than the 1911 mag. That's my chief gripe with the CCU, it'd be a lot more fun at the range if it used Thompson or Grease Gun mags.

As long as it wasn't a squib load, the bullet should be out of the barrel before the bolt starts moving backwards.

For some numbers, a Thompson bolt weighs just over a pound and a M3 Grease Gun bolt weighs 2 pounds. Both use fairly light recoil springs. I have a Mech-Tech CCU with a 1911 lower that also uses a 2 pound bolt and a 16" barrel. You can play with spring rates and buffers to come up with a system that doesn't batter itself to pieces.

p.s. If you're going with a .45 I'd suggest picking a higher capacity mag than the 1911 mag. That's my chief gripe with the CCU, it'd be a lot more fun at the range if it used Thompson or Grease Gun mags.

I was thinking a 1911 drum or extended mag. MAin thing being that it matches the mags one can use in a 1911.

Are you planing on basing it on the Feather AT-9? If so, I too have been considering building a carbine based off it. When I say based off it, I basically want to copy it. The tube receiver would be fairly easy to make. I think the hardest part would of course be the bolt. For a FCG you could just use one from an AR, ak, sks...etc...I've always thought adapting the 10/22 fcg to another rifle would be cool since you could just drop it in. Anyway, keep us posted, get your ass to work, and take lots of pics of the process.

I am just calling it an AT-9. It will be tube based of course but I am not really copying anything.

I am borrowing from some guns for ideas. Mostly from Bill Holmes, P.A Luty and some ideas I got from working on a friend's AK. Might use AK pivot pinsand retain them with snap rings.

The bolt is the only part I am really having a brain fart on. I have some ideas and have drawn up one of them. FCG will look like an open bolt FCG but I have figured a way to make it work closed. Another thing I learned from Bill Holmes' books.

What equipment do you have? Lathe and/or Mill? That will determine some of your design right there.

One of my initial design requirements is design simplicity and being able to use the least amount of specialty equipment. You know, files, hacksaw blades, drill press, hammers, measuring tools and knowledge.

We may be getting a 3 in 1 machine after taxes come in. Maybe.

But I don't want those things to be essential. They will just make it easier and better quality.